Magnitude of Electric Field at a Point

[tylerc1991]

Homework Statement

Given a sphere of radius R with a uniform charge distribution Q, what is the magnitude of the electric field at a point p0 inside of the circle?

Homework Equations

Flux = 4 pi r^2 E = Q/epsilon

The Attempt at a Solution

So I was watching an MIT lecture online (youtube is great for that), and the professor was doing this problem. When finding the magnitude of the electric field at a point inside of the sphere, he found it to be zero, which made perfect sense to me. Now studying for a test and reading through the book, I find this exact same problem, only now the magnitude of the electric field is given by a nonzero equation (kQr/R^3, where r is the distance from the center of the sphere to the point and r < R). (If anyone happens to have the book its 'Physics for Scientists and Engineers' vol 8 by Serway/Jewett, and the example is 24.3). So my question is this: who is correct? Is the magnitude of the electric field in fact 0 or is it related by the equation above?

 

[gneill]

In one case you have a conducting spherical shell with a given charge Q, evenly distributed over its surface. In your case you have a "solid" nonconducting sphere with charge Q distributed throughout.

Draw a Gaussian surface with some radius inside the sphere. What's the total charge contained therein?